This invention relates to a method and apparatus for the removal of boric acids or boron oxides from high-temperature stack gases to produce an effluent gas having boron and particulate levels with environmentally-acceptable properties.
Effluents from industrial processes, such as glass furnaces, can contain three types of air pollutants:
(a) dust, i.e. solid particulates of very small particle size
(b) permanent gases, e.g., SO.sub.2 or HF
(c) condensables, i.e. compounds which are in the gaseous state above a certain temperature, but which condense to a liquid or solid at lower temperatures.
Condensables, of which boron oxides or the corresponding acids are exemplary, are troublesome because, at high temperatures, they can pass through the particulate collection means of an effluent-treating system in the form of a vapor and condense in the atmosphere. Condensables characteristically form a detached plume, that is, a visible, persistent cloud which is dispersible only by dilution, at some distance from the stack.
All air quality standards, whether promulgated by state or national regulatory authorities set forth a requirement for particulate collection. This requirement can be met by fabric collectors (baghouses), electrostatic precipitators or scrubbers. Scrubbers are less than acceptable because the scrubber effluent may, without after-treatment, result in water pollution as a trade-off for air pollution. Accordingly, it may be preferred to collect particulate solids as a dry dust. For use of a baghouse, the particulates must be dry. When an electrostatic precipitator is used, it is preferred, but not absolutely required, that the particulates be dry.
Flue gases from glass making or melting furnaces, such as those used for the production of fiber glass, typically contain boron oxides and particulates, possibly admixed with small amounts of SO.sub.2, fluorides and nitrogen oxides.
In a prior art process for the removal of boron compounds from stack gases, disclosed by Teller in U.S. Pat. No. 3,995,005, it is taught that boron oxides exposed to moisture appear to hydrolyze to boric acid. Boric acid has a measurable vapor pressure at effluent gase temperatures generally considered acceptable. As a result, boric acid in gaseous form escapes from the recovery system and is exhausted to the atmosphere. In such systems a white plume of boric acid is formed by condensation of the effluent gases as the effluent stream hits the cooler atmosphere outside the treatment plant. It has been proposed by Teller to remove particulates and boron oxides by first cooling the effluent gases to about 170.degree.-175.degree. F. to condense boron oxides or boric acids and then removing the particulates by a baghouse filter.
The vapor pressure of boron hydrates and the vaporization mechanism of boron oxide hydrates has been elucidated by Petropavlovskii et al., abstracted in Chemical Abstracts, vol. 71, 116797k (1969) and vol. 72, 59386v (1970).
Removal of B.sub.2 O.sub.3 from exhaust gases of boron-containing fuels by wet scrubbing has been proposed by Hein et al., J. Air Pollution Control Association, vol. 11, 205-19 (1961), abstracted in Chemical Abstracts, vol. 55, 16964g (1961). A high collection efficiency was achieved, but the process was thought to be of limited industrial applicability because of high pressure loss and high ratio of scrubbing and cooling water to gas.
It will be apparent that present technology for the removal of boron compounds from stack gases is limited to methods in which boron acids or oxides are condensed at temperatures below about 300.degree. F. and that this technology requires large volumes of dilution air and consequently large apparatus for treating effluent streams. Moreover, the temperatures employed are below the dewpoint of corrosive components which may be contained in the effluent stream.